A. Literature
The open literature, which presents issues relevant to imaging systems in general, includes the following documents which are incorporated herein by reference:
1. Dan E. Dudgeon, "Fundamentals of Digital Array Processing," Proceedings of the IEEE, volume 65, pp. 898-904, June 1977.
2. Dan E. Dudgeon and Russell M. Mersereau, Multidimensional Digital Signal Processing, Chapter 6, Section 2: "Beamforming," Prentice Hall, 1984.
3. William C. Knight, Roger G. Pridham, and Steven M. Kay, "Digital Signal Processing for Sonar," Proceedings of the IEEE, volume 69, pages 1451-1506, November 1981. (Digital beamformers for use in sonar described on pages 1465-1471.)
4. Roger G. Pridham and Ronald A. Mucci, "A Novel Approach to Digital Beamforming," Journal of the Acoustical Society of America, volume 63, pages 425-434, February 1978.
5. Roger G. Pridham and Ronald A. Mucci, "Digital Interpolation Beamforming for Low-Pass and Bandpass Signals," Proceedings of the IEEE, volume 67, pages 904-919, June 1979.
6. P. Barton, "Digital Beamforming for Radar," IEE Proceedings, volume 127, part F, number 4, August 1980.
7. P. D. Carl, G. S. Kino, C. S. Desilets and P. M. Grant, "A Digital Synthetic Focus Acoustic Imaging System," Acoustic Imaging, volume 8, pp. 39-53, 1978.
8. B. D. Steinberg, "Digital Beamforming in Ultrasound," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, volume 39, pp. 716-721, November 1992.
9. Hans Steyskal, "Digital Beamforming Antennas," Microwave Journal, volume 30, No. 1, pp. 107-124, January 1987.
10. R. E. Crochiere and L. R. Rabiner, "Multirate Digital Signal Processing," Chapter 2, Prentice Hall, 1983.
B. Analog and Hybrid (Analog-Digital) Beamformer Systems
Relevant analog and hybrid (analog-digital) phased array beamformer system art can be found in the following patents which are incorporated herein by reference:
U.S. Pat. No.: Title: Inventor(s): 4,140,022 MULTIPLE Samuel H. Maslak TRANSDUCER ACOUSTIC IMAGING APPARATUS 4,550,607 PHASED ARRAY Samuel H. Maslak ACOUSTIC IMAGING J. Nelson Wright SYSTEM 4,699,009 DYNAMICALLY Samuel H. Maslak FOCUSED LINEAR Hugh G. Larsen PHASED ARRAY ACOUSTIC IMAGING SYSTEM 5,014,710 STEERED LINEAR Samuel H. Maslak and COLOR DOPPLER Donald J. Burch 5,165,413 IMAGING J. Nelson Wright Hugh G. Larson Donald R. Langdon Joel S. Chaffin Grant Flash, III
C. Digital Receive Beamformer Systems
The concept of a digital receive beamformer system has been proposed in the art with respect to ultrasound systems. By way of example, the following U.S. patents, all of which are incorporated herein by reference, discuss various aspects of such systems. The patents include:
 U.S. Pat. No.: Title: Inventor(s): 4,809,184 METHOD AND Matthew O'Donnell APPARATUS FOR Mark Magrane FULLY DIGITAL BEAM FORMATION IN A PHASED ARRAY COHERENT IMAGING SYSTEM 4,839,652 METHOD AND Matthew O'Donnell APPARATUS FOR HIGH William E. Engeler SPEED DIGITAL Thomas L. Vogelsong PHASED ARRAY Steven G. Karr COHERENT IMAGING Sharbel E. Noujaim SYSTEM 4,886,069 METHOD OF, AND Matthew O'Donnell APPARATUS FOR, OBTAINING A PLURALITY OF DIFFERENT RETURN ENERGY IMAGING BEAMS RESPONSIVE TO A SINGLE EXCITATION EVENT 4,893,284 CALIBRATION OF Mark G. Magrane PHASED ARRAY ULTRASOUND PROBE 4,896,287 CORDIC COMPLEX Matthew O'Donnell MULTIPLIER William E. Engeler 4,975,885 DIGITAL INPUT STAGE Dietrich Hassler FOR AN ULTRASOUND Erhard Schmidt APPARATUS Peter Wegener 4,983,970 METHOD AND Matthew O'Donnell APPARATUS FOR William E. Engeler DIGITAL PHASED John J. Bloomer ARRAY IMAGING John T. Pedicone 5,005,419 METHOD AND Matthew O'Donnell APPARATUS FOR Kenneth B. Welles, II COHERENT IMAGING Carl R. Crawford SYSTEM Norbert J. Plec Steven G. Karr 5,111,695 DYNAMIC PHASE William E. Engeler FOCUS FOR COHERENT Matthew O'Donnell IMAGING BEAM John T. Pedicone FORMATION John J. Bloomer 5,142,649 ULTRASONIC IMAGING Matthew O'Donnell SYSTEM WITH MULTIPLE, DYNAMICALLY FOCUSED TRANSMIT BEAMS 5,230,340 ULTRASOUND Theador L. Rhyne IMAGING SYSTEM WITH IMPROVED DYNAMIC FOCUSING 5,235,982 DYNAMIC TRANSMIT Matthew O'Donnell FOCUSING OF A STEERED ULTRASONIC BEAM 5,249,578 ULTRASOUND Sidney M. Karp IMAGING SYSTEM Raymond A. Beaudin USING FINITE IMPULSE RESPONSE DIGITAL CLUTTER FILTER WITH FORWARD AND REVERSE COEFFICIENTS
The basic feature of a digital receive beamformer system as disclosed above can include: (1) amplification of the ultrasound signal received at each element of an array such as, for example, a linear array; (2) direct per channel analog-to-digital conversion of the ultrasound signal with an analog-to-digital sampling rate at least twice the highest frequency in the signal; (3) a digital memory to provide delays for focusing; and (4) digital summation of the focused signals from all the channels. Other processing features of a receive beamformer system can include phase rotation of a receive signal on a channel-by-channel basis to provide fine focusing, amplitude scaling (apodization) to control the beam sidelobes, and digital filtering to control the bandwidth of the signal.
D. Transmit Beamforming
The above literature points out the ever-present desire to achieve more accurate focusing, better resolution, better sensitivities and higher frame rates in ultrasonic images. In order to do so, versatile adjustments of the beamforming characteristics are required in order to optimize the results for a given scan requirement. The greatest versatility is obtained when the ultrasound instrument can entirely change the number of beams transmitted simultaneously, the pulse waveform (PW) or continuous waveform (CW) characteristics, time delays and apodization values on a per-scan-line basis. However, such versatility can undesirably require extensive hardware resources if carried out in a direct implementation.
The above literature reveals extensive effort in the improvement of images through the use primarily of improved receive beamformers. Receive beamformers which employ digital techniques and digital signal processing have been reported in the prior art, though substantial improvements are still possible through innovative designs. Little effort, however, has been made to improve the characteristics of transmit beam formation. In the past, transmit beams were typically gated carrier pulses generated at a desired carrier frequency by analog circuitry. The only flexibility which was available to optimize the transmit pulse waveform shape (envelope) was typically an ability to specify the length of the pulse in terms of an integer number of carrier cycles it should contain, and some fixed analog filtering. Apodization and delay profiles for beamforming would be specified, and typically implemented in an analog fashion as well, with inherent precision limitations. The envelope shape of a pulse waveform was otherwise essentially fixed, and due to the limitations of analog processing, was not optimal. Also, whereas prior transmit beamformers were able to support different apodization profiles and different delay profiles for each firing in a scan, and were able to support different pulse lengths for each firing, the carrier frequency could not be changed between scan lines, nor could other characteristics of the envelope shape, other than its length, be modified.
There are significant advantages to be obtained by enhancing the flexibility of a transmit beamformer using digital processing techniques. For example, it would be desirable to be able to arbitrarily and independently shape the waveform which is to be applied to each of the ultrasonic transducer elements, in order to compensate for imperfections in the response of the transducer element or in the analog path to the transducer element. As another example, it would be desirable to change the waveform carrier frequency applied to each transducer element on a per-scan-line basis in order to mitigate the effects of grating lobes. See, for example, the above-cited METHOD AND APPARATUS FOR ADJUSTABLE FREQUENCY SCANNING IN ULTRASOUND IMAGING co-pending patent application. As yet another example, it would be desirable to improve focal precision in a transmit beam, such as by eliminating the tendency of analog components to drift over time or to correct for aberrating tissue (see, for example, the above-cited METHOD AND APPARATUS FOR REAL TIME, CONCURRENT ADAPTIVE FOCUSING IN AN ULTRASOUND BEAMFORMER IMAGING SYSTEM co-pending patent application). As still another example, it would be desirable to transmit pulses with special modulations, such as chirp or pseudo-random coded waveforms, in order to produce temporally longer pulses while maintaining range resolution in the resulting image. As yet another example, it would be desirable to be able to support multi-beam transmissions on a single firing as a way to increase the frame rate, to reduce speckle effects, or to achieve compound focusing (multiple focal points). As another example, it would be desirable to be able to transmit pulses having a precisely defined shape which will compensate for the distorting effects of attenuative body tissue. As yet another example, it would be desirable to update as many of the characteristics of a transmit pulse on a per-scan-line basis as possible. Simultaneous satisfaction of all these objectives cannot be obtained using presently available ultrasonic transmit beamformers.
Single channel digital programmable waveform generators are known in the field of test instruments for their ability to precisely generate arbitrary waveforms. See, for example, Priatko U.S. Pat. No. 4,881,190, and Tektronix, "Test and Measurement Product Catalog 1994", pp. 337-359, both incorporated herein by reference. The techniques used in these test instruments are generally not applicable or practical for phased array ultrasonic transmit beamforming, however, and in any event, have not been used for that purpose. For example, they could support only a single transmit channel and could not perform beamforming, in part because of cost, power and space constraints.
As used herein, an "analog" signal is a signal whose value at any given moment in time can take on any value within a continuous range of values. Analog signals can also be continuous in time, or sampled in time. A "digital" signal, as the term is used herein, can take on only discrete values at discrete time intervals. Also as used herein, the term "ultrasonic" refers to any frequency which is above the range of human hearing. Also as used herein, a device or function which is "programmable" includes those which can be programmed either by providing specific values for use by the device or function, or by selecting such values from a predetermined set of available values.